Conventionally, an uninterruptible power source includes: a converter configured to convert AC power from an AC power source into DC power; an inverter configured to convert (i) DC power generated by the converter or DC power of a power storage device into (ii) AC power and supply the AC power to a load; a bypass circuit including a thyristor switch and a contactor connected in parallel between the AC power source and the load; and a controller configured to control these.
In the uninterruptible power source, in a normal state in which AC power is normally supplied from the AC power source, the converter and the inverter are used, whereas in a power failure state in which the supply of AC power from the AC power source is stopped, supply of power is continued using the inverter. Such a type of circuit is called “on-line type”. In the on-line type, when the AC power source is normal and also when power failure takes place, power is supplied to the load using the inverter via a DC link, so that quality of output power is readily secured irrespective of quality of input power, thus attaining excellent stability of supply of power to the load. Meanwhile, the on-line type has a problem in terms of improvement in operation efficiency because power loss occurs due to energy passing through the converter and the inverter in the normal state.
In recent years, for increase in efficiency, an uninterruptible power source having an eco-mode has been proposed (for example, see U.S. Pat. No. 7,372,177 (Patent Document 1)). According to Patent Document 1, in the normal state, the uninterruptible power source performs bypass power supply to supply AC power from an AC power source to a load via a thyristor switch serving as a bypass circuit. Moreover, if a malfunction is detected in a bypass path during the execution of the bypass power supply, switching is performed from the bypass power supply to inverter power supply in which AC power is supplied from the inverter to the load.